Liquid crystal display devices have been used in watch and electronic calculator as well as in various measuring instrument, automobile panels, word processors, electronic notebooks, printers, computers and television sets more and more. As typical liquid crystal display systems there are employed TN (twisted nematic) system, STN (super twisted nematic) system, DS (dynamic light scattering) system, GH (guest host) system, FLC (ferroelectric liquid crystal) system, etc. Referring to driving system, multiplex driving system has been popular in place of conventional static driving system. Further, simple matrix system has been popular. In recent years, active matrix system has been put into practical use. Various properties have been required for liquid crystal materials depending on the display system or driving system. Thus, a large number of liquid crystal compounds have been synthesized so far.
Among these liquid crystal compounds, azine derivatives represented by the general formula (A): ##STR1## wherein R.sup.a represents an alkyl group have been known for a relatively long period of time. These liquid crystal materials are excellent as follows:
(i) The upper liquid crystal phase temperature limit T.sub.N-1 is high;
(ii) The chemical stability of these liquid crystal materials is relatively high; and
(iii) These liquid crystal materials can be prepared easily at a low cost.
However, the compounds (A) are disadvantageous in that they have a high melting point.
As a compound which can give solution to these problems there has been reported in JP-A-54-87688 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") an asymmetrical azine represented by the following general formula (B): ##STR2## wherein R.sup.a represents an alkyl group; and Rb represents an alkyl group different from R.sup.a. The compound (B) exhibits an upper liquid crystal phase temperature limit as high as the compound (A). In addition, the compound (B) exhibits a lower melting point than the compound (A).
Further, the above cited JP-A-54-87688 discloses an asymmetrical azine as a cyanobenzene derivative represented by the following general formula (C): ##STR3## wherein R.sup.a represents an alkyl group.
However, azine derivatives having alkenyl groups as side chains, azine derivatives containing trans-1,4-cyclohexylene group or fluorine-substituted 1,4-phenylene group, azine derivatives having fluorine atom or -OCF.sub.3 as a polar grouop which are expected to exhibit a lower viscosity or azine derivatives of three ring system which are expected to exhibit higher clearing point have never been known.
No processes for the preparation of an asymmetrical azine compound such as the foregoing compounds (B) and (C) have been established. For example, the above cited JP-A-54-87688 merely discloses an ordinary process for the preparation of azines but doesn't disclose detailed examples of preparation. According to the ordinary process for the preparation of azines, a hydrazone represented by the following general formula (A1): ##STR4## wherein R.sup.a is as defined above is reacted with a benzaldehyde derivative represented by the following general formula (A2): ##STR5## wherein R.sup.b is as defined above to prepare an azine.
However, during the reaction or post-treatment, disproportionation occurs to produce symmetrical azines represented by the following general formula (A3): ##STR6## wherein R.sup.a is as defined above or the following general formula (A4): ##STR7## wherein R.sup.b is as defined above in a relatively large amount as by-products besides the desired azines. Further, the preparation of the hydrazone (A1) is often liable to the production of symmetrical azines (A3) during the post-treatment. These symmetrical azines exhibit a good crystallinity and a high melting point similar to the foregoing compound (A) and thus can often hardly be separated and removed away by recrystallization. Further, these symmetrical azines have a poor miscibility with other liquid crystal compounds. Therefore, if the azine derivative thus obtained is used with these symmetrical azines left unseparated, these symmetrical azines can separate out in the composition. In particular, if as R.sup.a or R.sup.b there is used a cyano group or halogen atom, the properties of the product are drastically deteriorated, causing a big problem. Thus, in order to practically use the azines, it is preferred that the azines be free of symmetrical azines (A3) or (A4). Thus, preparation processes which produce no symmetrical azines as by-products are required.
In recent years, for the purpose of improving the response of STN-LCD, an active addressing driving system [Proc. 12th IDRC p. 503, 1992] and a multiline addressing driving system [SID '92 Digest, p. 232, 1992] have been proposed. These driving systems require liquid crystal materials having an elastic constant ratio K.sub.33 /K.sub.11 of about 1.5 and a relatively small dielectric anisotropy and viscosity as well as a great birefringence index in particular. Further, for the purpose of accomplishing a brighter display or a higher contrast ratio, a novel reflective type color liquid crystal display system utilizing the birefringence of liquid crystal and phase difference plate instead of a color filter layer [Technical Report of Society of Television Engineering, vol. 14, No. 10, p. 51, 1990] and a liquid display system having a reflective surface provided with small parabolic surfaces on the substrate electrode side have been proposed. These display devices require a liquid crystal material having birefringence properties which produce a greater phase difference due to difference in the wavelength of light or optical birefringence properties which maintain a high contrast even in a wide viewing angle. Further, for various purposes such as miniaturization, portability and increase of the number of pixels, liquid display devices having a broader operating temperature range have been required. These liquid display devices require a liquid crystal material having an elastic constant K.sub.11 of from 10 to 25, a good chemical stability and a broader nematic temperature and capable of shortening its switching time. Thus, a liquid crystal material having various physical properties which are comprehensively optimized has been required. The proposal of new liquid crystal compositions is still required.
Because of their excellent display quality, active matrix liquid crystal display systems have been on the market of portable terminals, flat panel display, projector, computer, etc. In the active matrix display system, TFT (thin film transistor), MIM (metal insulator metal) or the like is provided every pixel. In this system, emphasis has been placed on the high voltage holding ratio. Further, Kondo et al. proposed a super TFT combined with IPS mode to obtain an even wider viewing angle [Asia Display '95 Digest, p. 707, 1995] (The liquid crystal display device of these active matrix display systems will be hereinafter generically referred to as "TFT-LCD"). In order to cope with such a display device, new liquid crystal compounds or liquid crystal compositions have been still proposed as in JP-A-6-312949 and U.S. Pat. No. 5,480,581.
As a liquid crystal device requiring no polarizer or alignment treatment and producing a bright image with a high contrast there has been known a liquid crystal display device having polymer-encapsulated liquid crystal type. U.S. Pat. Nos. 4,605,284 and 4,435,047 propose gelatin, gum arabic, polyvinyl alcohol, etc. as capsulating substance. These liquid crystal display devices are also known in WO 8504262 and EP-A-205261. These liquid crystal display devices are disadvantageous in that the coincidence or discordance of the individual refractive index of liquid crystal material with the refractive index of polymer must be optimized and a voltage as high as not lower than 25 V is required to obtain a sufficient transparency.
A technique realizing low voltage driving properties, high contrast and multiplex driving properties required for liquid crystal display has been proposed in U.S. Pat. No. 5,304,323. These patents disclose a liquid crystal display device configured such that a liquid crystal material forms a continuous layer in which a polymer substance is distributed in a three-dimensional network.
Referring to liquid crystal materials relating to the foregoing object, European Patent 359,146 discloses a method for optimizing the birefringence index or dielectric anisotropy of liquid crystal materials and JP-A-6-222320 discloses a technique for identifying the elastic constant of liquid crystal materials. Further, U.S. Pat. No. 5,523,127 and EP-A-541912 disclose a liquid crystal material which comprises a fluoro compound to exhibit a contrast of about 40 when used in projection display. However, such a liquid crystal material finds difficulty in satisfying all the requirements, i.e., high resistivity, excellent voltage holding ratio, low driving voltage, strong light scattering that gives a high contrast ratio, fast switching time and good temperature characteristics. Thus, new proposals are still made.
In order to improve the foregoing liquid crystal display properties, a liquid crystal material having a desired birefringence index is required. Further, such a liquid crystal material also needs to have a higher chemical stability, a lower viscosity, a fast response time and a broader driving temperature range. The technique concerning the azine derivative of the general formula (I) of the present invention and its analogous compounds is referred to in, e.g., JP-A-54-87688. However, the knowledge of a mixture containing an azine derivative of the general formula (I) is not yet fully reported. Further, no liquid crystal materials or mixtures having properly adjusted birefringence index, dielectric anisotropy, elastic constant and other properties useful for TN-LCD, STN-LCD and TFT-LCD have been reported.
Further explaining, JP-A-54-87688 proposes the use of a compound represented by any one of the following general formulae (a-1) to (a-4): ##STR8## wherein R' and R" each independently represent an alkyl group or alkoxyl group as a compound having a positive dielectric anisotropy. However, the foregoing problems are left unsolved.